1. Field of the Invention
The present invention relates to a method of manufacturing a capacitive insulating film for a capacitor, and more particularly, to a method of forming a TiO2 film having a rutile crystal structure at low temperatures of 400° C. or less.
2. Description of the Related Art
As a semiconductor device such as DRAM device is finer, an insulating film for a capacitor having a high dielectric constant (capacitive insulating film) is needed.
As insulating material for a capacitor having a high dielectric constant, TiO2 (titanium oxide) may be exemplified. There are two types of crystal structures, i.e., anatase and rutile types in TiO2, which are well-known. The anatase crystal is a low temperature phase that is apt to be formed at low temperatures and has a low relative dielectric constant of about 40 or less. In the meantime, the rutile crystal is a high temperature phase that is typically formed at high temperatures and has a high relative dielectric constant of about 80 or more. In particular, when the rutile crystal is used as insulating material for a capacitor, it is possible to manufacture a high-capacity capacitor.
A TiO2 film can be formed by a variety of methods such as CVD (Chemical Vapor Deposition), ALD (Atomic Layer Deposition) and the like. When the TiO2 film is used for a semiconductor device, the ALD method is mainly used due to a standpoint of fining. However, when forming the TiO2 film to be used for a semiconductor device, it is difficult to form a rutile crystal of a high dielectric constant at low temperatures of 400° C. or less even with any method including the ALD method.
For example, according to the experiments performed by Gyeong Teak Lim et al. (Thin Solid Films 498 (2006) p 254-258), a TiO2 film is formed on silicon with the ALD method using precursor of TDMAT (tetrakis-(dimethylamino)titanium) and oxidizer of H2O. The TiO2 film becomes amorphous just after forming the film and is then crystallized by performing the annealing. The anatase crystals are generated with the annealing of 300° C. or more, the rutile and anatase crystals are generated at 700° C. or more and the rutile crystals become main at 800° C. or more. However, in a semiconductor process, it is difficult to perform the annealing at high temperatures, so as to avoid a bad effect on a semiconductor device such as transistor, as the device is gradually finer. Thus, it is actually impossible to perform the annealing at the high temperatures for obtaining the rutile crystals.
In addition, JP-A 2000-254519 discloses a technology of illuminating Ar ion beam so as to form a stacked structure of a rutile-type TiO2 film and an anatase-type TiO2 film for photocatalyst, thereby lowering a structure transition temperature from the anatase-type structure to the rutile-type structure. However, even with such means, the annealing of 500° C. or more should be performed so as to obtain a TiO2 film of the rutile crystal structure. In addition, when forming the TiO2 film in a location having a three-dimensional structure, such as a capacitor of a DRAM device, it is difficult to uniformly introduce the Ar ions by the ion illumination.
Additionally, JP-A 2007-110111 discloses a technology of forming a RuO2 film on a surface of a lower electrode for a capacitor, which electrode is made of Ru (ruthenium), thereby obtaining the rutile-type TiO2 film at low temperatures of 400° C. or less. However, since the material of the lower electrode is limited to Ru, it is difficult to change the material of the electrode and thus to form a capacitor having higher performance.
Thus, the inventors studied a method capable of forming a TiO2 film of a rutile crystal structure at temperatures as low as possible and easily forming a uniform TiO2 film without undergoing influences of electrode shapes and materials of a base even when it is applied to a capacitor of a three dimensional structure.
The inventors carried out an experiment of forming a TiO2 film using the ALD method. However, the anatase crystals were apt to be formed in a method of directly forming a TiO2 film. In addition, even when studying the annealing method, it was difficult to obtain a TiO2 film having rutile crystals only.
To be more specific, the TiO2 film formed at 200° C. was amorphous. When the amorphous film was crystallized with the annealing of high temperatures, both the anatase crystals and the rutile crystals were mixed. In addition, when the TiO2 film was formed at 250° C. or more, the anatase crystals were generated from the beginning. The anatase crystals once formed were stable and were not extinguished even when performing the annealing treatment of high temperatures at which the rutile crystals are generated. The anatase crystals are not desirable because they lower the dielectric constant.